chemengineering
Article
Energy and Resource E ffi cient Production of
Fluoroalkenes in High Temperature Microreactors
Konstantin Mierdel1,2,*, Andreas Jess3,4, Thorsten Gerdes2,5, Achim Schmidt2 and Klaus Hintzer6
1 Chair of Materials Processing, University Bayreuth, 95447 Bayreuth, Germany
2 Institut für innovative Verfahrenstechnik, InVerTec e. V., 95447 Bayreuth, Germany
3 Chair of Chemical Engineering, University Bayreuth, 95447 Bayreuth, Germany
4 Center of Energy Technology, University Bayreuth, 95447 Bayreuth, Germany
5 Chair of Ceramic Materials Engineering, Keylab Glasstechnology, 95447 Bayreuth, Germany
6 3M Dyneon, 84508 Burgkirchen, Germany
* Correspondence: Konstantin.mierdel@uni-bayreuth.de; Tel.:+49-921-55-7211
Received: 2 August 2019; Accepted: 17 September 2019; Published: 24 September 2019
Abstract:Tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are the most common monomers for the synthesis of fluoropolymers at industrial scale. Currently, TFE is produced via multistep pyrolysis of chlorodifluoromethane (R22), resulting in a high energy demand and high amounts of waste acids, mainly HCl and HF. In this study, a new chlorine-free process for producing TFE and HFP in a microreactor is presented, starting from partially fluorinated alkanes obtained from electrochemical fluorination (ECF). In the microreactor, high conversion rates of CHF3, which is used as a surrogate of partly fluorinated ECF streams, and high yields of fluoromonomers could be achieved. The energy saving and the environmental impact are shown by a life cycle assessment (LCA). The LCA confirms that the developed process has economical as well as ecological benefits, and is thus an interesting option for future industrial production of fluoroalkenes.
Keywords: microreactor; fluoropolymers; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); life cycle assessment (LCA)
1. Introduction
Fluoropolymers are high-tech materials with extraordinary properties (non-flammable, chemically stable, high dielectric strength, and operating temperatures up to 280◦C), and thus are not substitutable in numerous technical applications, for example, in the semiconductor/electronic industry, seal- and corrosion-resistant applications, or in future technologies for energy conversion like fuel cells [1]. The worldwide consumption of fluoropolymers was about 270,000 t per year in 2015 [2]. The most common polymer is polytetrafluoroethylene (PTFE) with 140,000 t a−1. The complete fluoropolymer production requires about 162,000 t of tetrafluoroethylene (TFE) and 41,000 t of hexafluoropropylene (HFP) per year [2]. In addition, partly fluorinated monomers like vinylidene fluoride (VDF) (43,000 t a−1) and vinyl fluoride (VF) are necessary for polyvinylidene fluoride (PVDF) production, but also for different applications, such as fluorinated membranes or new binding systems in electrodes for lithium ion batteries [3,4].
Industrial production:In contrast to “simple” monomers such as ethylene or propylene, which are produced in a one-step process (steamcracking), the production of fluoromonomers follows an expensive, multistage chemical chlorine-synthesis, which is called the R22 route [5,6]. The R22 route was first reported by Downing and Benning in 1945 [7]. This synthesis, as depicted in Table1for TFE, starts with the partial chlorination of methane to trichloromethane (Table1, Equation (1)). In
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this reaction step, not only trichloromethane, but also mono- and dichloromethane are produced, which can only finally be converted into trichloromethane by a respective recycle to the reactor. The other synthesized by-product, tetrachloromethane, cannot be used in further reactions and must be incinerated and disposed of. Further, large amounts of HCl (three mol HCl per mol methane) are formed in this reaction step. In the second synthesis step (Table1, Equation (2)), trichloromethane and hydrogen fluoride, which is produced by the reaction of calcium fluoride and sulphuric acid, are converted to difluorochloromethane (R22) through the use of an antimony chloride catalyst. The third reaction step (Table1, Equation (3)) consists of the pyrolysis of R22 to difluorocarbene and HCl at temperatures between 800◦C and 900◦C [8]. The gained difluorocarbene reacts immediately to tetrafluoroethylene (TFE) by dimerization (Table1, Equation (4)).
Table 1.Reaction equations of the industrial R22 route.
Reaction ∆rH298[kJ/mol] Equation
CH4+3Cl2 450◦C
→ CHCl3+3HCl −305 Equation (1)
CHCl3+2HF SbCl→5 CHClF2+2HCl 168 Equation (2) CHClF2 800
◦C
→ :CF2+HCl 120 Equation (3)
2 :CF2 800
◦C
→ C2F4 −295 Equation (4)
A closer look at the reaction steps in Table1illustrates that current TFE production leads to high amounts of unwanted by-products and waste acids, especially hydrochloric acid (HCl) and hydrofluoric acid (HF). A drawback is also the contamination of HCl with low amounts of HF (around 1500 ppm), which makes further use or recycling of the acid impossible. In addition, current TFE production has a high energy demand for the separation, purification, recycling, or incineration of by-products. It is thus highly desirable to develop a chlorine-free and more energy efficient process to produce fluoromonomers.
Alternative reaction routes:The first invention of a chlorine-free synthesis of TFE was made in 1957 and involved an electric arc or a plasma process for the reaction of CF4with coal particles to TFE.
However, both processes need huge amounts of energy and achieve only low yields of TFE, and thus could not compete with the conventional R22 route [9,10].
Another approach for the production of fluoromonomers is the pyrolysis of fluoroform (R23), which is a by-product through over fluorination of trichloromethane (Equation (2)) in the R22 route.
The annual amount of this waste stream is about several millions of kg per year [11]. Pyrolysis of CHF3 is conducted by Hauptschein and Fainberg [12] at temperatures of 700 to 960◦C, residence times about 0.6 s to about 0.08 s, and atmospheric pressure. They reached high conversion rates of about 80% and yields of C2F4and C3F6of about 58.1%. Additionally, Gelblum et al. [11] tries to utilize CHF3waste streams by co-pyrolysing R23/R22 mixtures in the range of 690 to 775◦C and contact times less than 2 s.
In the sole feed pyrolysis of R23 at 775◦C, Gelblum et al. measured conversion rates of CHF3below 1% and detected TFE as only product gas.
In contrast to R22, which was economically favorable earlier because of its availability as a general-purpose refrigerant, a new approach for a complete chlorine-free process is based on electro-chemical fluorination (ECF), where perfluorinated hydrocarbons are formed by reaction of anhydrous hydrofluoric acid (AHF) with the corresponding hydrocarbons. State-of-the-art ECF processes only provide partly- and perfluorinated alkanes, but no alkenes like TFE [13]. Therefore, an additional process step is necessary. Aschauer et al. [14] showed that high yields of TFE could be obtained in a microwave plasma, when the feed consists of perfluorinated alkanes like C2F6and C4F10, which originated from ECF, as described by Schmeiser et al. or Nagase et al. (Table2) [15,16].
The problem of utilizing feed gases from ECF is the low selectivity to perfluorinated products obtained in this process. Depending on the hydrocarbon used as feed in the ECF, up to 70% of the product spectrum consists of only partly fluorinated alkanes (Table2) [15]. Production of 1000
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t perfluorinated compounds results in waste streams of about 400 t of partly fluorinated residues.
Further, hydrofluorocarbon by-products, for example, CHF3, C2HF5, and C2H2F4, are produced, which are not allowed to be released into the atmosphere, because of their high global warming potential (GWP) (Table2) [17].
Table 2. Product distribution of electro-chemical fluorination (ECF) [16,18] and global warming potential (GWP) of fluorocarbons [17].
Feed Reaction Total Current
Eff. a[%] Product Comp. [mol%] 100 yr GWP [kg CO2/kg]
CH4 CH4+4HF→CF4+4H2 48.2
CF4 24.1 6500
CHF3 11.0 11,700
CF2H2 8.3 650
CFH3 56.6 150
C2H6 C2H6+6HF→C2F6+6H2 65.0
C2F6 56.2 9200
C2F5H 14.2 2800
C2F4H2 12.1 1000
CF3CFH2 10.3 1300
C2F3H3 7.2 300
aThe calculation of current efficiency was based on the amount of current assumed to be necessary for forming fluorine with a discharging fluoride ion, which would react with the sample [18].
Currently, all partly fluorinated compounds must be incinerated (Table3) under very high expenditure of energy. Another drawback of incineration is HF formation, which requires neutralization by bases like NaOH or Ca(OH)2to receive depositable salts (NaF, CaF2) (Table3).
Table 3.Reaction equation of incineration and pyrolysis of CHF3.
Reaction Equation
CHF3incineration
CHF3+12O2 →3HF+CO2 Equation (5) CH4+2O2 →2H2O+CO2 Equation (6) Ca(OH)2+2HF →2H2O+CaF2 Equation (7)
CHF3pyrolysis
CHF3→ HF+:CF2 Equation (8) 2 :CF2→ C2F4 Equation (9) C2F4+ :CF2 →C3F6 Equation (10)
To counteract the disadvantages of thermal utilization and to utilize the complete product spectrum of the ECF, in this study, partly fluorinated alkanes were proven regarding their suitability as starting material for fluoromonomer production. The respective process route of a utilization of partly fluorinated compounds for the production of TFE and HFP is illustrated in Figure1. The synthesis starts with the supply of feed gas as by-products of ECF followed by pyrolysis for generation of monomers like TFE and HFP. The main advantages of this process are the utilization of cheap starting materials (currently waste streams) and the prevention of HCl waste acids.
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Figure 1. Design of a two-step chlorine-free process for fluoromonomer production.
In the field of fluorine chemistry and fluorine materials, the handling and processing of different reactions in microreactors are of growing relevance. The reasons for this trend are safer handling of hazardous reactants, a reduction in the (potential) exposure to toxic or hazardous chemical species, and higher process safety in terms of thermal runaway by efficient cooling. Moreover, the application of microreactors in fluorine containing reactions is beneficial in terms of reduced reaction volume, optimal process conditions, and thus higher yields of target products [19,20]. Scale-up of a microreactor process can be easily done just by numbering up [21]. Microreactors are also advantageous with regard to kinetic studies of highly exothermic or endothermic reactions, for example, here for the highly endothermic decomposition of fluorinated hydrocarbons. Compared with lab-scale fixed bed reactors, microreactors have a much higher surface-to-volume ratio, which enables very efficient heating or cooling, and thus almost isothermal operation. Hence, the determination of kinetic parameters is not hindered by limitations of heat and mass transfer [22].
In this work, the experimental results of the pyrolysis of partly fluorinated alkane CHF3 in a high temperature microreactor are presented. On the basis of the rate of CHF3 conversion, the kinetic data of this initial reaction step of pyrolysis (dehydrofluorination of CHF3 to HF and difluorocarbene) were determined. In consecutive reactions, fluorinated products such as tetrafluoroethylene (TFE), hexafluoroproplene (HFP), and perfluoroisobutene (PFiB) are formed. The measured product distribution was compared with a simulation of the pyrolysis using kinetic data from literature for these consecutive steps. The optimum reaction conditions for monomer formation were investigated.
Finally, the ecological and economical potential of the process was evaluated by a life cycle assessment (LCA).
2. Materials and Methods
2.1. Design of the Microreactor for the Pyrolsis of CHF3
Pyrolysis of partly fluorinated alkanes for the production of fluoromonomers and determination of kinetic parameters was carried out in a high temperature siliconcarbide (SiC) microreactor provided by 3M technical ceramics (Table 4).
Table 4. Technical data of SiC-microreactor.
Material Properties Unit Value
Density g/cm3 >3.15
Porosity % <2.0
Grain size µm 2–10
Thermal conductivity λSiC at 25 °C W/(m K) 130 Specific heat cSiC at 25 °C J/(g K) 0.69
Figure 1.Design of a two-step chlorine-free process for fluoromonomer production.
In the field of fluorine chemistry and fluorine materials, the handling and processing of different reactions in microreactors are of growing relevance. The reasons for this trend are safer handling of hazardous reactants, a reduction in the (potential) exposure to toxic or hazardous chemical species, and higher process safety in terms of thermal runaway by efficient cooling. Moreover, the application of microreactors in fluorine containing reactions is beneficial in terms of reduced reaction volume, optimal process conditions, and thus higher yields of target products [19,20]. Scale-up of a microreactor process can be easily done just by numbering up [21]. Microreactors are also advantageous with regard to kinetic studies of highly exothermic or endothermic reactions, for example, here for the highly endothermic decomposition of fluorinated hydrocarbons. Compared with lab-scale fixed bed reactors, microreactors have a much higher surface-to-volume ratio, which enables very efficient heating or cooling, and thus almost isothermal operation. Hence, the determination of kinetic parameters is not hindered by limitations of heat and mass transfer [22].
In this work, the experimental results of the pyrolysis of partly fluorinated alkane CHF3in a high temperature microreactor are presented. On the basis of the rate of CHF3conversion, the kinetic data of this initial reaction step of pyrolysis (dehydrofluorination of CHF3to HF and difluorocarbene) were determined. In consecutive reactions, fluorinated products such as tetrafluoroethylene (TFE), hexafluoroproplene (HFP), and perfluoroisobutene (PFiB) are formed. The measured product distribution was compared with a simulation of the pyrolysis using kinetic data from literature for these consecutive steps. The optimum reaction conditions for monomer formation were investigated.
Finally, the ecological and economical potential of the process was evaluated by a life cycle assessment (LCA).
2. Materials and Methods
2.1. Design of the Microreactor for the Pyrolsis of CHF3
Pyrolysis of partly fluorinated alkanes for the production of fluoromonomers and determination of kinetic parameters was carried out in a high temperature siliconcarbide (SiC) microreactor provided by 3M technical ceramics (Table4).
Table 4.Technical data of SiC-microreactor.
Material Properties Unit Value
Density g/cm3 >3.15
Porosity % <2.0
Grain size µm 2–10
Thermal conductivityλSiCat 25◦C W/(m K) 130 Specific heat cSiCat 25◦C J/(g K) 0.69
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The ceramic microreactor is depicted in Figure2and has a channel cross section of 1 mm2and a reaction volume of 1.2 mL. The microreactor design consists of four sections. An in- and outlet for the supplied and produced gases, a meander reaction zone, and a quenching zone. The microreactor has beneficial material properties like high thermal conductivity and specific heat, which provides almost isothermal operation, and thus an accurate measurement of the kinetics of CHF3pyrolysis and of the subsequent fluoromonomer formation, respectively.
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The ceramic microreactor is depicted in Figure 2 and has a channel cross section of 1 mm2 and a reaction volume of 1.2 mL. The microreactor design consists of four sections. An in- and outlet for the supplied and produced gases, a meander reaction zone, and a quenching zone. The microreactor has beneficial material properties like high thermal conductivity and specific heat, which provides almost isothermal operation, and thus an accurate measurement of the kinetics of CHF3 pyrolysis and of the subsequent fluoromonomer formation, respectively.
Figure 2. Microreactor used for the experiments: (1) gas inlet, (2) reaction zone, (3) thermocouple guide tube, (4) quenching system, and (5) product gas outlet.
2.2. Measurement Setup for Pyrolsis of CHF3
The setup for the determination of conversion of partly fluorinated substances is schematically depicted in Figure 3. It consisted of the gas supply, the high temperature micro reactor, the quenching with a following exhaust gas treatment, and the online gas chromatography (GC) analysis.
CHF3 was supplied by Air Liquide with purities of 99.95% and N2 by Sauerstoffwerke Friedrichshafen GmbH with purities of 99.9%. Mass flow controllers from Bronkhorst (EL-FLOW F- 201CV) delivered the gases. The volumetric flow rate of fluoroform was varied from 0.25 to 5 mL min−1. N2 was used as inert carrier gas to adjust a volumetric content of 10% fluoroform in the feed stream. The microreactor was placed in a tubular electric furnace and heated to the desired constant temperature. The reaction temperature was monitored by one thermocouple placed in the meander shaped reaction zone (see Figure 2). After passing through the microreactor, the product gas was quickly quenched below 100 °C to avoid consecutive reactions of TFE to di- or other oligomers.
Additionally, hydrogen fluoride (HF) formed through the pyrolysis was neutralized with KOH (40 g/L) during passage through a washing bottle. The HF-concentrations were determined by pH probe, Knick SE555X/1-NMSN, placed in the washing bottles. The moisture behind the washing column was removed via molecular sieve of 0.3 nm (Metrohm Ion analysis). For the detection of the product gas distribution, an online gas chromatography (GC) HP 6890 equipped with a thermal conductivity detector (TCD) and a Carbopack C33 column (6 m, 5% picric acid) was used. The GC was operated in the temperature range from 40 to 110 °C, with a temperature ramp of 10 K/min and a dwell time of 5 min at 110 °C. The detector ran at a temperature of 200 °C and the injection temperature was 150
°C.
Figure 2.Microreactor used for the experiments: (1) gas inlet, (2) reaction zone, (3) thermocouple guide tube, (4) quenching system, and (5) product gas outlet.
2.2. Measurement Setup for Pyrolsis of CHF3
The setup for the determination of conversion of partly fluorinated substances is schematically depicted in Figure3. It consisted of the gas supply, the high temperature micro reactor, the quenching with a following exhaust gas treatment, and the online gas chromatography (GC) analysis.ChemEngineering 2018, 2, x FOR PEER REVIEW 6 of 20
Figure 3. Flow sheet of the measurement setup for the pyrolysis of partly fluorinated alkanes.
2.3. Pyrolysis Reaction, Kinetic Modeling, and Life Cycle Assessment (LCA) for the Pyrolsis of CHF3
Experimental procedure: Before each pyrolysis experiment, the microreactor was purged with N2. The microreactor was then heated to the desired reaction temperature under N2 atmosphere. After reaching the target temperature, CHF3 was supplied. The pyrolysis experiments were conducted at temperatures of 775–875 °C, residence times of 0.4–2.5 s, a molar ratio of CHF3 to N2 of 0.1, and at almost atmospheric pressure (Table 5).
Table 5. Experimental parameters of CHF3 pyrolysis.
Parameters Unit Range
Temperature °C 775–875
Pressure mbar 1013–1263 (absolute)
Flow rate (standard temperature and pressure (STP)) mL/min 7.5–50
Residence time s 0.4–2.5
Modelling of CHF3 pyrolysis: Several researchers have already investigated the thermal decomposition of fluoroform (CHF3) over a broad temperature and residence time range, as well as with shockwave techniques or turbulent reactors. The main products in the pyrolysis of CHF3 reported in literature are the valuable monomers TFE and HFP, but also undesired partly fluorinated alkanes and alkenes like C2F6, C2HF3, C3F8, and C2F5H, as well as toxic compounds like i-C4F8 and HF [23]. It is generally agreed that the initial step in the synthesis of fluoromonomers is the highly endothermic decomposition of fluoroform yielding difluorocarbene (:CF2) [24]. Hence, an efficient supply of heat is needed to keep the dehydrofluorination going. The decomposition of CHF3 starts up at 700 °C and is intensified by an increase of temperature and residence time. Reported activation energies for the decomposition of CHF3 are in the range of 244–295 kJ/mol [24,25]. By dimerization of the generated difluorocarbene, the main product TFE is formed, but dimerization of difluorocarbene is reversible under reaction conditions, which is experimentally evidenced by Couture et al. [26] in their studies to produce HFP out of TFE. The yield of TFE passes a maximum value at a certain reaction (residence) time in consequence of the consecutive reaction with difluorocarbene to HFP [27]. Thus, the formation of fluoromonomers by pyrolysis of fluoroform depends on the generation and consumption of difluorocarbene. The complete reaction network of CHF3 pyrolysis, which consists of seven reactions, is described in detail in Section 3.3.
In this study, the calculation of the CHF3 conversion was done based on the kinetic parameters (pre-exponential factor, activation energy) determined by respective own experiments. The rates of formation of the consecutive products tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and perfluoroisobutene (PFiB) were calculated based on the respective kinetic parameters reported in the
Figure 3.Flow sheet of the measurement setup for the pyrolysis of partly fluorinated alkanes.
CHF3 was supplied by Air Liquide with purities of 99.95% and N2 by Sauerstoffwerke Friedrichshafen GmbH with purities of 99.9%. Mass flow controllers from Bronkhorst (EL-FLOW F-201CV) delivered the gases. The volumetric flow rate of fluoroform was varied from 0.25 to 5 mL min−1. N2was used as inert carrier gas to adjust a volumetric content of 10% fluoroform in the feed stream. The microreactor was placed in a tubular electric furnace and heated to the desired constant temperature. The reaction temperature was monitored by one thermocouple placed in the
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meander shaped reaction zone (see Figure2). After passing through the microreactor, the product gas was quickly quenched below 100◦C to avoid consecutive reactions of TFE to di- or other oligomers.
Additionally, hydrogen fluoride (HF) formed through the pyrolysis was neutralized with KOH (40 g/L) during passage through a washing bottle. The HF-concentrations were determined by pH probe, Knick SE555X/1-NMSN, placed in the washing bottles. The moisture behind the washing column was removed via molecular sieve of 0.3 nm (Metrohm Ion analysis). For the detection of the product gas distribution, an online gas chromatography (GC) HP 6890 equipped with a thermal conductivity detector (TCD) and a Carbopack C33 column (6 m, 5% picric acid) was used. The GC was operated in the temperature range from 40 to 110◦C, with a temperature ramp of 10 K/min and a dwell time of 5 min at 110◦C. The detector ran at a temperature of 200◦C and the injection temperature was 150◦C.
2.3. Pyrolysis Reaction, Kinetic Modeling, and Life Cycle Assessment (LCA) for the Pyrolsis of CHF3
Experimental procedure:Before each pyrolysis experiment, the microreactor was purged with N2. The microreactor was then heated to the desired reaction temperature under N2atmosphere. After reaching the target temperature, CHF3was supplied. The pyrolysis experiments were conducted at temperatures of 775–875◦C, residence times of 0.4–2.5 s, a molar ratio of CHF3to N2of 0.1, and at almost atmospheric pressure (Table5).
Table 5.Experimental parameters of CHF3pyrolysis.
Parameters Unit Range
Temperature ◦C 775–875
Pressure mbar 1013–1263 (absolute)
Flow rate (standard temperature and pressure (STP)) mL/min 7.5–50
Residence time s 0.4–2.5
Modelling of CHF3 pyrolysis: Several researchers have already investigated the thermal decomposition of fluoroform (CHF3) over a broad temperature and residence time range, as well as with shockwave techniques or turbulent reactors. The main products in the pyrolysis of CHF3 reported in literature are the valuable monomers TFE and HFP, but also undesired partly fluorinated alkanes and alkenes like C2F6, C2HF3, C3F8, and C2F5H, as well as toxic compounds like i-C4F8and HF [23]. It is generally agreed that the initial step in the synthesis of fluoromonomers is the highly endothermic decomposition of fluoroform yielding difluorocarbene (:CF2) [24]. Hence, an efficient supply of heat is needed to keep the dehydrofluorination going. The decomposition of CHF3starts up at 700◦C and is intensified by an increase of temperature and residence time. Reported activation energies for the decomposition of CHF3are in the range of 244–295 kJ/mol [24,25]. By dimerization of the generated difluorocarbene, the main product TFE is formed, but dimerization of difluorocarbene is reversible under reaction conditions, which is experimentally evidenced by Couture et al. [26] in their studies to produce HFP out of TFE. The yield of TFE passes a maximum value at a certain reaction (residence) time in consequence of the consecutive reaction with difluorocarbene to HFP [27].
Thus, the formation of fluoromonomers by pyrolysis of fluoroform depends on the generation and consumption of difluorocarbene. The complete reaction network of CHF3pyrolysis, which consists of seven reactions, is described in detail in Section3.3.
In this study, the calculation of the CHF3conversion was done based on the kinetic parameters (pre-exponential factor, activation energy) determined by respective own experiments. The rates of formation of the consecutive products tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and perfluoroisobutene (PFiB) were calculated based on the respective kinetic parameters reported in the literature. The differential equations describing both the CHF3conversion and the product yields at different residence times and temperatures were solved by Matlab R2015b using ode 45 solver.
Life cycle analysis (LCA): The evaluation of the ecological and economical potential of the developed process (LCA) was carried out according to ISO 1404X standards with the software Simapro
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8.3.0 and the Database ecoinvent 3.3. The assessment criteria are the cumulative energy demand (CED) in version V1.09 and the 18 impact categories of the ReCiPe method midpoint (H) v1.12/European ReCiPe (H).
3. Results
3.1. Calculation of Conversion and Yield in the Pyrolysis of CHF3 The pyrolysis of CHF3can be regarded as a first order reaction:
rCHF3 =−dcCHF3
dτ =kCHF3cCHF3. (11)
The reaction rate r (mol m−3s−1) is defined here based on the residence time (τ) in the tubular reactor and depends on the rate constantkCHF3(1/s) and the concentration of CHF3(mol m−3). For an isothermal and ideal plug flow reactor, integration of Equation (11) yields the following conversion:
XCHF3 =1−e−kCHF3τ. (12)
The assumption of a plug flow reactor is only valid in good approximation for a highly turbulent flow (Reynolds number Re>2300). For the microreactor used in this study, Re is defined as
Re= u dhyd
ν , (13)
whereuis the gas velocity (at reaction conditions),dhydis the characteristic (hydraulic) diameter, andν is the kinematic viscosity. For a quadratic channel with height and width s (both here 1 mm),dhydis given by the following (A: cross sectional area;C: circumference):
dhyd= 4·A C = 4s
2
4s =s. (14)
For the given microreactor,Reis very small, for example, 23 for a typical residence time of 0.4 s and temperature of 875◦C. Thus, the flow is strictly laminar under the applied conditions in this study.
For laminar flow, the conversion of CHF3in a tubular reactor is usually calculated as follows [28]:
Xlam=1−
1−Da 2
·e−Da2 −Da2 4
∞
Z
Da 2
e−x
x dx, (15)
Da=kCHF3(T)·τ, (16)
kCHF3(T) =kCHF3,0e−EART, (17) τ= .Vreactor
VFeed(T)
= Lreaction zone
uFeed(T) . (18)
Dais the so-called Damköhler number. Solutions of the integral in Equation (15) are tabulated [29,30].
In the case of laminar flow, the assumption of plug flow behavior is not valid and usually a lower conversion (as determined by Equation (15)) is achieved compared with an ideal plug flow reactor (PFR) (Equation (12)). However, the advantage of the small dimensions (above all the small diameter) of the used microreactor is a relatively fast diffusion in the radial direction, that is, the characteristic
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time for radial diffusion,τD, is much shorter than the average residence time (τ) [28]. In Equation (19), rtis the radius of reaction channel, andDmolis the diffusivity of gas (N2).
τD= r
2 t
Dmol τ= Lreaction zone
uFeed(T) . (19)
In this study, the residence timeτ(0.4–2.5 s) is always higher than the characteristic time of diffusionτD(0.1 s, determined by Equation (19)). In this case, we may use the axially dispersed plug flow model and the so-called axial dispersion coefficientDax, respectively, to inspect the deviation from an ideal PFR (see, for example, the work of [28]):
Dax
uLt
= 1
Bo = Dax udt
dt
Lt, (20)
Dax=Dmol+ u
2d2t
192Dmol. (21)
In the Equations (20) and (21),uis the gas velocity, anddtandLtare the diameter and length of the reactor, respectively. TheBodensteinnumberBorepresents the ratio of convective flux to diffusive flux [28]:
Bo= uLt
Dax. (22)
ForRehigher than 10, the first term on the right-hand side of Equation (21) (Dmol) can be neglected and theBoas a measure for the deviation from plug flow behavior reads as follows:
Bo=192 Lt
u
Dmol
4r2t
=192τ 1 4τD
≈50 τ τD
(f or Re>10). (23) In general, plug flow is almost reached ifBo>80 [28]. In the used microreactor,Bois always>175 (875◦C, 0.4 s). Thus, Equation (12) is valid and was used to calculate the conversion of CHF3.
The yields and selectivities ofTFEandHFPare calculated by the residual molar rates in mol/s (reactor outlet) and the initial molar rate of CHF3(Equations (24)–(27)). The factors 2 and 3 consider the stoichiometry of the respective reactions.
YTFE =2 n.TFE
n.CHF3,0
, (24)
YHFP =3 n.HFB
n.CHF3,0
, (25)
STFE =2
n.TFE
n.CHF3,0−n.CHF3
= YTFE
XCHF3, (26)
SHFP =3
n.HFB
n.CHF3,0−n.CHF3
= YHFP
XCHF3. (27)
For the first order assumption, the rate constant is given (based on Equation (12)) by the following:
kCHF3=−ln
1−XCHF3
τ
. (28)
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The pre-exponential factork0and the activation energyEAwere determined by theArrhenius equation (Equation (29) and the respective plot based on experiments at different temperatures (775 to 875◦C) and residence times (0.35 to 2.5 s). All experiments were conducted at atmospheric pressure.
ln(kCHF3) =ln(kCHF3,0)−EA R
1
T. (29)
3.2. Kinetic Parameters, Conversion, and Product Distribution in the Pyrolysis of CHF3
Kinetic parameters:The experimental data are fitted with a least square fitting for the calculation of the kinetic parameters. The data are shown in Figure4a and the straight lines are in good agreement with the first order approach. The values ofChemEngineering 2018, 2, x FOR PEER REVIEW kCHF3,0andEAare listed in Figure4b. 9 of 20
(a) (b)
Figure 4. Measurement of kinetic parameters in the pyrolysis of CHF3: (a) determination of k(T) at different temperatures, (b) Arrhenius plot for determination of EA and kCHF3,0.
First order assumption and conversion: The measured CHF3 conversion for different concentrations of fluoroform is shown in Figure 5 (a) for three temperatures. The conversion is constant for each temperature, which is a clear indication of a first order reaction.
(a) (b)
Figure 5. (a) Conversion of CHF3 for different temperatures and (b) comparison of simulated and measured conversion of CHF3 over temperature for different residence times.
In Figure 5 (b), the conversion of CHF3 is shown as a function of temperature for different residence times. The calculated CHF3 conversion (dashed lines) matches the experimental data (dots) quite well (for the assumption of a first order reaction). Measured conversion of about 25% at 875 °C and 0.4 s is in good agreement with CHF3 conversion of 27.3% in the experiments of Hauptschein et al. [12].
Product distribution: The products of CHF3 pyrolysis are practically only C2F4 and C3F6. C2F6
and i-C4F8 are only formed in traces at the highest measured temperatures. The yields and selectivities of TFE and HFP (as valuable products) at shortest and longest adjusted residence time are shown in Figure 6 and 7 in a temperature range of 775 to 875 °C. With increasing temperature and residence time, the yield and selectivity, respectively, of the consecutive product HFP (C3F6) increases. In this study, a maximum yield of 23% TFE (0.4 s at 875 °C) and of 17% TFE and 35% HFP (2.5 s at 875 °C) is achieved. Compared with yields of Hauptschein et al. [12], the generated yields of TFE and HFP are in the same range. Thus, with the same process parameters (880 °C and 0.4 s), the calculated yield by
0.0 0.5 1.0 1.5 2.0 2.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2
1.4 775°C 800°C 825°C 850°C 875°C
ln(1/(1-X)) / -
Residence time τ / s 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8
0.01 0.1 1
CHF3 EA: 194 kJ/mol k0: 3.84E+08 1/s
k(T) / s-1
Reciprocal temperature / 10-4 K-1 750 800
850 900
Temperature / °C
700 750 800 850 900
0 10 20 30 40 50 60
XCHF3 / %
Temperature / °C
cCHF3 10%
cCHF3 20%
cCHF3 50%
τ: 1.7-2.3 s p: 30-40 mbar (rel.)
600 700 800 900 1000
0 20 40 60 80 100
cCHF3: 10%
Sim.
Meas.
XCHF3 /%
Temperature / °C
0.4s 0.8s 2.5s
Figure 4.Measurement of kinetic parameters in the pyrolysis of CHF3: (a) determination of k(T) at different temperatures, (b) Arrhenius plot for determination ofEAandkCHF3,0.
First order assumption and conversion: The measured CHF3 conversion for different concentrations of fluoroform is shown in Figure 5a for three temperatures. The conversion is constant for each temperature, which is a clear indication of a first order reaction.
ChemEngineering 2018, 2, x FOR PEER REVIEW 9 of 20
(a) (b)
Figure 4. Measurement of kinetic parameters in the pyrolysis of CHF3: (a) determination of k(T) at different temperatures, (b) Arrhenius plot for determination of EA and kCHF3,0.
First order assumption and conversion: The measured CHF3 conversion for different concentrations of fluoroform is shown in Figure 5 (a) for three temperatures. The conversion is constant for each temperature, which is a clear indication of a first order reaction.
(a) (b)
Figure 5. (a) Conversion of CHF3 for different temperatures and (b) comparison of simulated and measured conversion of CHF3 over temperature for different residence times.
In Figure 5 (b), the conversion of CHF3 is shown as a function of temperature for different residence times. The calculated CHF3 conversion (dashed lines) matches the experimental data (dots) quite well (for the assumption of a first order reaction). Measured conversion of about 25% at 875 °C and 0.4 s is in good agreement with CHF3 conversion of 27.3% in the experiments of Hauptschein et al. [12].
Product distribution: The products of CHF3 pyrolysis are practically only C2F4 and C3F6. C2F6
and i-C4F8 are only formed in traces at the highest measured temperatures. The yields and selectivities of TFE and HFP (as valuable products) at shortest and longest adjusted residence time are shown in Figure 6 and 7 in a temperature range of 775 to 875 °C. With increasing temperature and residence time, the yield and selectivity, respectively, of the consecutive product HFP (C3F6) increases. In this study, a maximum yield of 23% TFE (0.4 s at 875 °C) and of 17% TFE and 35% HFP (2.5 s at 875 °C) is achieved. Compared with yields of Hauptschein et al. [12], the generated yields of TFE and HFP are in the same range. Thus, with the same process parameters (880 °C and 0.4 s), the calculated yield by
0.0 0.5 1.0 1.5 2.0 2.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2
1.4 775°C 800°C 825°C 850°C 875°C
ln(1/(1-X)) / -
Residence time τ / s 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8
0.01 0.1 1
CHF3 EA: 194 kJ/mol k0: 3.84E+08 1/s
k(T) / s-1
Reciprocal temperature / 10-4 K-1 750 800
850
900 Temperature / °C
700 750 800 850 900
0 10 20 30 40 50 60
XCHF3 / %
Temperature / °C
cCHF3 10%
cCHF3 20%
cCHF3 50%
τ: 1.7-2.3 s p: 30-40 mbar (rel.)
600 700 800 900 1000
0 20 40 60 80 100
cCHF3: 10%
Sim.
Meas.
XCHF3 /%
Temperature / °C
0.4s 0.8s 2.5s
Figure 5. (a) Conversion of CHF3for different temperatures and (b) comparison of simulated and measured conversion of CHF3over temperature for different residence times.
In Figure5b, the conversion of CHF3is shown as a function of temperature for different residence times. The calculated CHF3conversion (dashed lines) matches the experimental data (dots) quite well
ChemEngineering2019,3, 77 10 of 19
(for the assumption of a first order reaction). Measured conversion of about 25% at 875◦C and 0.4 s is in good agreement with CHF3conversion of 27.3% in the experiments of Hauptschein et al. [12].
Product distribution:The products of CHF3pyrolysis are practically only C2F4and C3F6. C2F6
and i-C4F8are only formed in traces at the highest measured temperatures. The yields and selectivities of TFE and HFP (as valuable products) at shortest and longest adjusted residence time are shown in Figures6and7in a temperature range of 775 to 875◦C. With increasing temperature and residence time, the yield and selectivity, respectively, of the consecutive product HFP (C3F6) increases. In this study, a maximum yield of 23% TFE (0.4 s at 875◦C) and of 17% TFE and 35% HFP (2.5 s at 875◦C) is achieved. Compared with yields of Hauptschein et al. [12], the generated yields of TFE and HFP are in the same range. Thus, with the same process parameters (880◦C and 0.4 s), the calculated yield by Equations (24) and (25) of Hauptschein et al. is 17.9% for TFE and 6.6% for HFP. The highest yield of 58.1% is reached at 960◦C and 0.3 s. The maximum yield of fluoromonomers (52%) is obtained in this study at lower temperatures (about 100◦C) and longer residence times (2.5 s), which could be explained by the reversible dimerization of TFE to difluorocarbene and the consecutive reaction to HFP at the adjusted residence times [26].
ChemEngineering 2018, 2, x FOR PEER REVIEW 10 of 20
Equations (24) and (25) of Hauptschein et al. is 17.9% for TFE and 6.6% for HFP. The highest yield of 58.1% is reached at 960 °C and 0.3 s. The maximum yield of fluoromonomers (52%) is obtained in this study at lower temperatures (about 100 °C) and longer residence times (2.5 s), which could be explained by the reversible dimerization of TFE to difluorocarbene and the consecutive reaction to HFP at the adjusted residence times [26].
(a) (b)
Figure 6. Measured product formation in the pyrolysis of CHF3: (a) yield of C2F4 and (b) yield of C3F6 (line: guide for the eye, dot: experimental data).
(a) (b)
Figure 7. Selectivity of fluoromonomers TFE (a) and HFP (b) in the pyrolysis of CHF3 (line: guide for the eye, dot: experimental data).
Long-term process stability of CHF3-pyrolysis in the SiC-microreactor: After the evaluation of suitable process parameters, the CHF3 pyrolysis was carried out at residence times of 0.4 and 2.5 s for several hours to study the “long-term” microreactor stability and performance. Figure 8 shows that isothermal conditions and a stable reactor operation were reached in both cases after about 1 h. The time of 1 h is necessary for heating the oven to target temperatures and purging all tubes and washing bottles with the same concentration of reaction products.
775 800 825 850 875
0 10 20 30 40
cCHF3: 10%
YTFE / %
Temperature / °C 0.4s
2.5s
775 800 825 850 875
0 10 20 30 40
cCHF3: 10%
YHFP /%
Temperature / °C 0.4s
2.5s
750 775 800 825 850 875 900
0 20 40 60 80 100
STFE / %
Temperature / °C 0.4s
2.5s
750 775 800 825 850 875 900
0 20 40 60 80 100
SHFP / %
Temperature / °C 0.4s
2.5s
Figure 6.Measured product formation in the pyrolysis of CHF3: (a) yield of C2F4and (b) yield of C3F6
(line: guide for the eye, dot: experimental data).
ChemEngineering 2018, 2, x FOR PEER REVIEW 10 of 20
Equations (24) and (25) of Hauptschein et al. is 17.9% for TFE and 6.6% for HFP. The highest yield of 58.1% is reached at 960 °C and 0.3 s. The maximum yield of fluoromonomers (52%) is obtained in this study at lower temperatures (about 100 °C) and longer residence times (2.5 s), which could be explained by the reversible dimerization of TFE to difluorocarbene and the consecutive reaction to HFP at the adjusted residence times [26].
(a) (b)
Figure 6. Measured product formation in the pyrolysis of CHF3: (a) yield of C2F4 and (b) yield of C3F6 (line: guide for the eye, dot: experimental data).
(a) (b)
Figure 7. Selectivity of fluoromonomers TFE (a) and HFP (b) in the pyrolysis of CHF3 (line: guide for the eye, dot: experimental data).
Long-term process stability of CHF3-pyrolysis in the SiC-microreactor: After the evaluation of suitable process parameters, the CHF3 pyrolysis was carried out at residence times of 0.4 and 2.5 s for several hours to study the “long-term” microreactor stability and performance. Figure 8 shows that isothermal conditions and a stable reactor operation were reached in both cases after about 1 h. The time of 1 h is necessary for heating the oven to target temperatures and purging all tubes and washing bottles with the same concentration of reaction products.
775 800 825 850 875
0 10 20 30 40
cCHF3: 10%
YTFE / %
Temperature / °C 0.4s
2.5s
775 800 825 850 875
0 10 20 30 40
cCHF3: 10%
YHFP /%
Temperature / °C 0.4s
2.5s
750 775 800 825 850 875 900
0 20 40 60 80 100
STFE / %
Temperature / °C 0.4s
2.5s
750 775 800 825 850 875 900
0 20 40 60 80 100
SHFP / %
Temperature / °C 0.4s
2.5s
Figure 7.Selectivity of fluoromonomers TFE (a) and HFP (b) in the pyrolysis of CHF3(line: guide for the eye, dot: experimental data).
Long-term process stability of CHF3-pyrolysis in the SiC-microreactor:After the evaluation of suitable process parameters, the CHF3pyrolysis was carried out at residence times of 0.4 and 2.5 s for several hours to study the “long-term” microreactor stability and performance. Figure8shows that
ChemEngineering2019,3, 77 11 of 19
isothermal conditions and a stable reactor operation were reached in both cases after about 1 h. The time of 1 h is necessary for heating the oven to target temperatures and purging all tubes and washing bottles with the same concentration of reaction products.ChemEngineering 2018, 2, x FOR PEER REVIEW 11 of 20
(a) (b)
Figure 8. Time on stream: (a) production of C2F4 (0.4 s) and (b) production of C2F4 and C3F6 (2.5 s) at 875 °C.
3.3. Simulation of Product Distribution
The gas phase pyrolysis of CHF3 at temperatures between 775 and 875 °C was examined in more detail using both experimental data and modeling techniques. The first step in the pyrolysis of CHF3
is dehydrofluorination to hydrogen fluoride (HF) and difluorocarbene (:CF2); see Equation (8) [24,31,32]. Similar to the reaction of R22, the generated difluorocarbene dimerizes immediately to TFE [33]. The formation of HFP—Equation (10)—takes place by consecutive reaction of TFE with another CF2 carbene. The overall reaction network can be described by a series of consecutive reactions listed in Table 6. The kinetic parameters for CHF3 decomposition were experimentally determined in this study. For the remaining consecutive reactions, literature references were used (Table 6).
The kinetic parameters of the reaction of C2F4 (TFE) with a difluorocarbene radical (CF2) to C3F6
(HFP) were calculated by fitting k4(T) for each temperature and by the Arrhenius plot. Compared with the study of Ainagos [34], EA is in the same range, but k0 is lower (values in Table 6). The reaction network was then finally simulated with the kinetic parameters listed in Table 6. The comparison of experimental and simulated data is shown in Figure 9 for constant pyrolysis temperatures of 850 °C and 875 °C and a residence time up to 3 s. The measured and simulated (dimensionless) concentrations of the educt CHF3 and the products TFE, HFP, and PFiB are in good agreement.
Table 6. Kinetic parameters of the consecutive reaction network.
Reaction kn
(T) k0 [s−1 or s−1M−1] EA [kJ/mol] Literature
𝐶𝐻𝐹 → 𝐻𝐹+: 𝐶𝐹 k1 4.16 × 108 195 this study
2 : 𝐶𝐹 → 𝐶 𝐹 (𝑇𝐹𝐸) k2 8.71 × 108 0 Poltanskii [35]
𝐶 𝐹 → 2 : 𝐶𝐹 k3 4.57 × 1016 293 Poltanskii [35]
𝐶 𝐹 + : 𝐶𝐹 → 𝐶 𝐹 (𝐻𝐹𝑃) k4 4.97 × 108 122 this study (calc.
from data)
𝐶 𝐹 → 𝐶 𝐹 + : 𝐶𝐹 k5 1.91 × 1013 333 Poutsma. [36]
𝐶 𝐹 + : 𝐶𝐹 → 𝑖 − 𝐶 𝐹 (𝑃𝐹𝑖𝐵) k6 1.20 × 1016 385 Bauer [37]
0 100 200 300 400 500 600 700 0
200 400 600 800 1000
Temperature / °C
Time on stream / min
0 20 40 60 80 100
TFE
XCHF3, YTFE /%
CHF3
0 120 240 360 480 600
0 200 400 600 800 1000
TFE
Temperature / °C
Time on stream / min CHF3
HFP
0 20 40 60 80 100
XCHF3, YTFE, YHFP /%
Figure 8.Time on stream: (a) production of C2F4(0.4 s) and (b) production of C2F4and C3F6(2.5 s) at 875◦C.
3.3. Simulation of Product Distribution
The gas phase pyrolysis of CHF3at temperatures between 775 and 875◦C was examined in more detail using both experimental data and modeling techniques. The first step in the pyrolysis of CHF3is dehydrofluorination to hydrogen fluoride (HF) and difluorocarbene (:CF2); see Equation (8) [24,31,32]. Similar to the reaction of R22, the generated difluorocarbene dimerizes immediately to TFE [33]. The formation of HFP—Equation (10)—takes place by consecutive reaction of TFE with another CF2carbene. The overall reaction network can be described by a series of consecutive reactions listed in Table6. The kinetic parameters for CHF3decomposition were experimentally determined in this study. For the remaining consecutive reactions, literature references were used (Table6).
The kinetic parameters of the reaction of C2F4(TFE) with a difluorocarbene radical (CF2) to C3F6 (HFP) were calculated by fittingk4(T) for each temperature and by the Arrhenius plot. Compared with the study of Ainagos [34],EAis in the same range, butk0is lower (values in Table6). The reaction network was then finally simulated with the kinetic parameters listed in Table6. The comparison of experimental and simulated data is shown in Figure9for constant pyrolysis temperatures of 850◦C and 875◦C and a residence time up to 3 s. The measured and simulated (dimensionless) concentrations of the educt CHF3and the products TFE, HFP, and PFiB are in good agreement.
Table 6.Kinetic parameters of the consecutive reaction network.
Reaction kn(T) k0[s−1or s−1M−1] EA[kJ/mol] Literature
CHF3→ HF+:CF2 k1 4.16×108 195 this study
2 :CF2→ C2F4(TFE) k2 8.71×108 0 Poltanskii [35]
C2F4 → 2 :CF2 k3 4.57×1016 293 Poltanskii [35]
C2F4+ :CF2 →C3F6(HFP) k4 4.97×108 122 this study (calc. from data)
C3F6 →C2F4+ :CF2 k5 1.91×1013 333 Poutsma. [36]
C3F6+ :CF2 → i−C4F8(PFiB) k6 1.20×1016 385 Bauer [37]
